This invention relates to a method and apparatus for preparing thermoplastic foams.
Various methods have been employed for preparing the thermoplastic foams. In many of these methods, an extruder has been used. The reasons for this are that, when an extruder is used, the thermoplastic foams can be prepared continuously, and hence advantageously. In a method using an extruder, a thermoplastic resin is heated and melted in the extruder, a foaming agent is injected into the molten resin to form a mixture, in which the foaming agent is uniformly mixed with the resin under pressure, the mixture is then forced through a die to have a desired shape in cross section, and thereafter extruded from the die. Thus, extruded resin is foamed in the air to be a foam.
In the method mentioned above, in order to prepare a uniform composition by injecting the foaming agent into the resin in the extruder, it is required that the resin be at a very high temperature and in a sufficiently molten state. However, when the resin containing the foaming agent is extruded out of the die, the resin should not be at such a high temperature. That is, when extruded from the die the resin must be softened enough that it may be foamed, but must not be at such a high temperature that it cannot maintain its shape. Thus, in the method for preparing the foamed article by means of the extruder, the resin containing the foaming agent should be cooled on the way to the die.
This cooling must be carried out equally and uniformly throughout the resin mass because the resin composition containing the foaming agent cannot be foamed uniformly unless the composition has a uniform temperature. For this reason, various methods and apparatuses have been proposed to uniformly cool the molten resin composition. Many of the apparatuses are constructed having a rotating member which can be heated and cooled provided in a barrel, which can be also heated and cooled. The rotating member generally has on its outer surface a number of blades, each of which projects towards inner surface of the barrel and extends substantially in the axial direction of the rotating member. The blades are located in a zigzag pattern in the axial direction.
Japanese Patent Publication No. 48-544 teaches the use of flat plates for the blades and that the plates should be secured to outer surface of the rotating member so that the surface of each of the blades may lie in a tangential plane of the outer surface of the axis. The publication describes these tangential blades as "rotating wings," and mentions that perforations may be provided in the plates. Japanese Patent Publication 54-42026 discloses that the blades should be made of flat plates, which project perpendicularly on outer surface of the rotating member, and further suggests that the blades should be provided with perforations which are inclined towards either outwardly or inwardly by turns in every other one of the blades.
Uniform cooling can seemingly be attained when a mixer formed by a barrel and a member rotatably provided in the barrel is used when the rotating member is further provided with blades, each of which projects on outer surface of the member and extends in the axial direction, and when cooling is carried out at least from a barrel side of the apparatus as mentioned above. However, in case wherein a large amount of resin was extruded with the purpose of obtaining a foamed article having a large cross section, it was not possible to uniformly foam both the surface portion and inner portion of the article when such a mixer was used. Thus further improvement is required in order to manufacture a uniform foam having a large cross-sectional area by an extrusion method with high efficiency.
Aside from the cooling mixer mentioned above, it is known that a zigzag mixer may be used for cooling the resin. This is disclosed, for example, in Japanese Laid-Open Patent Application No. 53-148755. The Laid-Open Patent Application mentions that a zigzag mixer having a specific structure may be used for cooling a molten resin. This structure essentially comprises a conduit with tortuous passages formed therein. However, when a foamable molten resin was passed through the specific zigzag mixer and cooled from outside thereof by circulating a cooling medium into a conduit, as taught by the publication, the resin passed through the mixer tended to be unevenly foamed having a low density in inner portion thereof and a high density in outer surface portion thereof. Moreover, the thus obtained product includes shear lines appearing much like weld lines at places in the product where plates in the zigzag mixer were positioned. These shear lines or weld lines were considered to be drawbacks in quality of the product. Therefore, it was found that when the zigzag mixer was used for cooling and mixing the molten foamable resin, the resin could be cooled but that a uniformly cooled and mixed foam having a large cross section could not be obtained.
Moreover, it is also known that a zigzag mixer can be directly mounted on the forward end of an extruder for simply mixing a resin, i.e. without using the mixer for cooling the resin. This is disclosed, for example, in Japanese Laid-Open Patent Application No. 52-133364. However, in case wherein the zigzag mixer was directly mounted on the forward end of the extruder according to the teachings of the Laid-Open Application, efficient and sufficient cooling could not be attained, and it was not possible to foam the foamable resin uniformly.